Before I started window cleaning I suffered for years with a lower bad back and sciatica caused by disc prolapses. I even had an operation which was only partly successful. I’ve only ever done wfp and when I first started 8 years ago it was hard going and I felt like giving up many times. But I kept going, gradually building up my round and before long I wasn’t getting any pain at all and I have had no back problems since.
I can only put this down to the muscles in my back being strengthened and general daily excersise you get from wfp cleaning.
My advice to to anyone getting back pain would be to take some time off and rest till the pain is reduced, Don’t try to carry on with painkillers as this can make things worse. Gentle stretching excersises help a lot also (loads of advise online about which excersises are best)